Rob,
I checked the Vigor documentation and it states that you can assign separate VLANs (Virtual LAN's) to each of the Vigor's 4 switch ports. What it doesn't tell you is whether each VLAN can have its own IP sub-net assigned to it and a separate DHCP scope set up, which is what you really need to achieve this so you don't have to do a lot of configuration. You could leave each cottage instructions on setting their network cards up with a static IP address but this may be too difficult for some users. Also, if you have business users staying this may be a problem for them as most businesses lock down the network cards and don't allow staff to change the settings.
If you are going to supply PC's for each cottage, manually set the IP address of each PC in each cottage so that you know which address is being used in which cottage.
The Draytek website www.draytek.co.uk/products/vigor2800.html states that the router has "Pre-set DHCP/MAC binding, with optional strict bind". Not exactly sure what this is or what it can do for you but have a look at this in your manual or on the website and it might give you some way of doing what you want.
Without knowing whether the PC's are going to be yours or not its hard to advise you in any more detail.
Good luck, Reg99